1. Field of the Invention
The present invention relates to modulation systems for controlling and coordinating the rate of fuel consumption and delivery of combustion air to gas fired, pressurized combustion appliances.
2. Description of the Prior Art:
Gas or oil-fired equipment modulation can be defined as the control and coordination of both the rate of fuel consumption and the delivery of combustion air to a gas or oil-fired appliance. Such appliances include, for example, water heaters and boilers of the type utilized for commercial/industrial use, as well as for residential use, furnaces, and the like. The objective of the modulation system for such appliances is the maintenance of a consistent air/fuel ratio that results in both good combustion, i.e. minimization of the generation of carbon monoxide, and good efficiency, i.e., the minimization of excess air consistent with good combustion. Prior art modulation systems are known in the art which include three principal components; a modulation drive, a fuel valve and a means to control the rate of combustion air entering the system. Typically, these components are interconnected by a system of linkages that permits adjustment of the air/fuel ratio.
In operation, the known input modulation systems allow the combustion burner to initiate a firing sequence at a minimum rate which promotes smoother ignition and reduced thermal shock. Control of the input rate of air/fuel being supplied to the burner is usually provided by a signal from a differential thermostat. Thus, the greater the difference between a set point of the thermostat (usually the desired output water temperature) and the actual temperature of the stored water, the greater the input rate of the burner. As the temperature differential diminishes, the input rate is proportionately reduced. This behavior represents a key benefit of modulation, i.e., the ability to more closely match input to demand thus reducing the likelihood of short cycling.
The practical input range of a conventional modulation system is limited by flame stability at reduced input. As the rates of fuel and air are reduced, turbulence is also reduced which eventually results in poor mixing and unacceptable combustion, i.e., high carbon monoxide generation. Typically this condition limits the input range of power burners to about 3 or 4 to 1.
The present invention has as its object to provide a high ratio modulation system which substantially increases the input range of power burners for gas fired, pressurized appliances to 10 to 1, or more, by introducing a means to better stabilize the burner flame pattern at a reduced air/fuel input rate.
While the invention will be described with respect to a high efficiency, gas fired water heater, it will be understood that the same principles apply to other gas fired, pressurized appliances such as boilers, furnaces, and the like.